1. Field of the Invention
The present invention relates to a negative electrode for lithium secondary battery which comprises a negative electrode material and a negative electrode collector having the negative electrode material attached to a surface thereof, and to a lithium secondary battery employing such a negative electrode.
2. Description of the Related Art
As one type of the new secondary batteries featuring high powers and high energy densities, a lithium secondary battery is in wide use, which employs a non-aqueous electrolyte and which is adapted for charge/discharge by way of transfer of lithium ions between a positive electrode and a negative electrode.
In such a lithium secondary battery, the negative electrode comprises a negative electrode material including a material to be alloyed with lithium, and a negative electrode collector having the negative electrode material attached thereon.
However, in a case where the negative electrode material includes the material to be alloyed with lithium, the charging/discharging processes involve substantial expansion/contraction of the material to be alloyed with lithium. Hence, with repeated charging/discharging processes, the negative electrode is gradually deformed. A thin lithium secondary battery such as of a card type encounters a problem of increase in the thickness thereof.
More recently, the above lithium secondary batteries have come into use as a power source for a variety of portable devices. This leads to a demand for a lithium secondary battery having an even higher energy density.
In this connection, there has been proposed a lithium secondary battery, such as disclosed in Japanese Patent Publication No. 2948205, which comprises a negative electrode fabricated as follows. A composite powder material containing silicon or a silicon-carbon composite material, having a high capacity density, is used as the negative electrode material. A slurry containing the negative electrode material and a binder resin is applied to the negative electrode collector of a metal foil, which is sintered in a non-oxidizing atmosphere to form the negative electrode.
Unfortunately, if a negative electrode collector of a metal foil having a smooth surface is processed by the method wherein the slurry containing the negative electrode material and binder resin is applied to the negative electrode collector and sintered in the non-oxidizing atmosphere, the following problem results. That is, the negative electrode collector of the smooth surface has such a small contact area with the negative electrode material that the sintering process cannot achieve a significant increase in the adhesion between the negative electrode collector and the negative electrode material. As a result of the expansion or contraction of the negative electrode material in association with the charging/discharging processes, contact between the negative electrode material and the negative electrode collector is decreased. Thus, resistance between the negative electrode material and the negative electrode collector is increased so that a battery is decreased in charge/discharge cycle performance.